Nanoengineered TiO
2
and Ta-doped TiO
2
for highly conducting transparent electrodes and
photoanodes
P. Mazzolini
1,2
, C.S. Casari
1,2
, V. Russo
1
, G. Gregori
3
, D. Chrastina
4
, R. Ferragut
4
, S. Nakao
5
,
T. Hitosugi
6
, A. Li Bassi
1,2
*
*
1
Dipartimento di Energia, Politecnico di Milano, via Ponzio 34/3, 20133 Milano (Italy)
2
CNST - Center for Nano Science and Technology @PoliMi, Istituto Italiano di Tecnologia, via Pascoli
70/3, 20133 Milano (Italy)
3
Max Planck Institute for Solid State Research, Heisenbergstr. 1, 70569 Stuttgart (Germany)
4
L-NESS, Dipartimento di Fisica, Politecnico di Milano - Polo Territoriale di Como, Via Anzani 42, 22100
Como (Italy)
5
Kanagawa Academy of Science and Technology (KAST), Kawasaki 213-0012, Japan
6
Advanced Institute for Materials Research (AIMR), Sendai 980-8577, Japan
Donor doped TiO
2
-based Transparent Conducting Oxides (TCO) [1] are receiving increasing
attention because of the particular properties of TiO
2
, such as photoactivity or chemical stability in
reducing atmospheres; the use of TiO
2
as an electron selective layer or photoanode in
electrochemical or perovskite solar cells motivates the goal of controlling functional properties in
these TiO
2
-based materials.
We here demonstrate the possibility to finely tune the electrical and optical properties of TiO
2
and Ta-doped TiO
2
(TaTO) thin films by engineering their structure and morphology at the
nanoscale. TiO
2
and TaTO films were synthesized at room T via pulsed laser deposition, followed
by thermal annealing to obtain conducting polycrystalline anatase. The best functional properties
(resistivity ~5×10
-4
Ωcm, transmittance in the visible >80% for a 150 nm TaTO film) were
obtained when depositing at 1 Pa O
2
followed by vacuum annealing at 550°C [2].
We also demonstrate the possibility to crystallize the films exploiting an ultra-fast thermal
treatment at ambient pressure in N
2
, yielding virtually the same conductivity as a conventional
anneal carried out in vacuum (for doped and undoped TiO
2
). By monitoring the crystallization
threshold via in-situ electrical measurements, we were able to investigate the effects of O
incorporation on the electrical properties [3]. This process, which is industrially scalable, reduces
the annealing time from about 3 hours to a few minutes and allows the electrical properties to be
uncoupled from the influence of the annealing environment.
The strong sensitivity of electrical and optical properties to the deposition and annealing
atmospheres reveals that the defect chemistry of donor doped TiO
2
is not trivial and involves a
strict interplay between extrinsic dopant atoms, oxygen vacancies and ‘electron killer’ defects such
as Ti vacancies and O interstitials. We exploited a combination of different techniques such as X-
ray diffraction, Raman spectroscopy and positron annihilation spectroscopy in order to characterize
the material structure, understand and control the relation between structure, defects and functional
properties. In particular, a clear and unexpected dependence of the E
g
(1) anatase Raman peak shift
and the electron carrier density was found and will be discussed.
Finally, by increasing the O
2
pressure during the deposition process we are able to obtain
hierarchically nanostructured mesoporous layers, which could effectively act as large surface area
photoanodes with tunable functional properties. This opens the way to the realization of an all-TiO
2
transparent electrode/selective layer/photoanode with a reduced number of interfaces and thus of
recombination centers, which could be beneficial for electron transport in real devices.
1. T. Hitosugi
et al
., Jpn. J. Appl. Phys. 44, L1063 (2005)
2. P. Mazzolini et al., J. Phys. Chem. C 119, 6988 (2015)
3. P. Mazzolini et al., Adv. Electr. Mater. (2016),
doi: 10.1002/aelm.201500316
I 18
-110-
